add pkg-config integration

[dvzrv]

Providing build or runtime dependencies for a software is usually taken care of in a package management system on Linux distributions.
During build time pkg-config (or more recently the drop-in replacement pkgconf) is used by all major build systems (e.g. autotools, cmake, meson) to find dependencies, their versions, components, etc.

To make use of the vst3sdk on Linux operating systems (and also on macOS) in a useful way, an install target for files required by hosts and plugins should be defined and a pkg-config integration (a .pc file) be added.
A common location would be e.g. /usr/include/vst3sdk (in a packaging context) to host the *.cpp and *.h files from e.g. the base, pluginterfaces and public.sdk submodules.

A pkg-config integration (vst3sdk.pc) might then look like this:

Name: vst3sdk
Description: VST 3 Plug-In SDK
Version: 3.7.0_build_116
Cflags: -I/usr/include/vst3sdk
Libs:

A common location for that file is /usr/lib/pkgconfig/.

[Be-ing]

To be completely clear, there is no conflict between installing pkgconfig and CMake config files. It is fairly easy to do both. For example, take a look at libkeyfinder.